Process for producing molded parts by polymerization of lactams in molds

ABSTRACT

A method for manufacturing a sheet molded part by polymerization of lactams in a sheet mold includes the steps of placing a fiber composite material into a sheet mold having large plane surfaces and fixedly positioning the fiber composite material in the sheet mold. Subsequently, the lactams are introduced into the sheet mold and the lactams are polymerized.

BACKGROUND OF THE INVENTION

The invention relates to a process for manufacturing molded parts bypolymerization of lactams in molds with addition of fiber compositematerial. The invention relates furthermore to novel molded partsproduced according to the inventive process.

The technical manufacture of molded parts from lactams, preferably byactivated alkaline quick polymerization or block polymerization has beenin use for approximately 30 years. The term molded parts in the contextof the present invention refers to elements that can be produced withthe known casting and polymerization techniques and which includesemi-finished products, shaped elements etc.

In the field of machine and apparatus construction, for example, it hasbeen attempted for many years to replace sheet metal with othermaterials that have technical and/or financial advantages in theirmanufacture and/or their application. As alternative materials, amongothers, plastic materials have found acceptance which have versatileproperties and can be matched very precisely to respective requirements.However, to date no plastic material has been found with which canreplace metals completely with respect to strength, stability, hardness,stiffness, tenacity, temperature resistance, behavior in fire etc.Insofar as individual plastic materials have at least partially reachedthe technological properties of metal or in some cases even surpassthem, their use is, in general, not comparable to the use of metals withrespect to economic considerations. It is thus still necessary to selectfor each application the respective optimal replacement plastic materialfrom a great palette of possible variants in order to comply optimallywith the respective profile of specifications.

Fields for the use of sheet plastic molded parts are the automobileindustry and transportation. In these fields sheet molded parts are usedwhich in the past have been made from sheet metal. In the recent past,these molded parts have been replaced in particular by thermosettingmaterials, whereby especially unsaturated polyester resins (UP resins),polyurethanes (PU resins), and epoxide resins (EP resins) have beenpopular. For mechanically minimally loaded parts, thermoplasticmaterials have been used more and more whereby mostly the mass-producedplastics polypropylene (PP), polycarbonate (PC), acrylonitrile butadienestyrene (ABS), and different mixtures are being used.

The plurality of employed plastic materials, especially in theautomobile industry, has been considered more and more as a disadvantagein comparison to the metallic materials because in the recent past aneed for recycling and reuse of the discarded old apparatus hasdeveloped. While the metal recycling problem has been solved for yearssubstantially by scrap metal treatment, plastic recycling is still inthe start-up phase. It has been shown already that a sorting accordingto plastic types is very complicated and expensive. Instead, it seams tobe more useful to employ from the beginning only a small number ofplastic materials. These plastic materials should then be used for therespective applications in large quantities, in the automobile industry,for example, for bumpers, side panels, fenders, roofs, the underbodygroups, engine compartment hood etc. In this context, it is desirable toprovide only one plastic material for a large number of applications.

Important parameters for recycling of plastics is the required energyexpenditure and the reworking degree of the remaining plastic to newplastic parts. Conventionally, the plastic parts are comminuted andforeign materials are cleaned off. The resulting raw material is, ingeneral, available in granular form. When a thermoplastic material isprovided it can be used completely in injection molding machines toproduce new parts. However, thermosetting plastics do not melt undercomparable conditions and can only be used partially as a fillermaterial when new thermosetting parts are produced. Thus, thethermoplastic materials due to their complete recyclability have becomemore and more important in comparison to thermosetting materials so thatmore emphasis has been placed on the search for suitable thermoplasticreplacement materials.

A known thermoplastic material with an especially balanced propertyspectrum is polyamide which is already being used for manufacturing highquality technical parts. Polyamides can be affected with respect totheir technological properties so that they are interesting as areplacement for metals with respect to technical and economicconsiderations. However, thermoplastics could not be used in the pastfor producing sheet molded parts, i.e., parts that have thin wallsrelative to their surface area. Especially for supporting parts, whichmust withstand strong mechanical forces without breaking, thermosettingelements in addition to sheet metal still dominate.

Molded parts of polyamide can be produced, for example, bypolymerization of lactams. The technical manufacture of molded partsfrom lactams, preferably by activated alkaline quick polymerization orblock polymerization, has been used for approximately 30 years. The termsheet molded parts in the context of the present invention includesconventionally produced molded parts made of foil, sheet metal, or otherlarge surface area materials which conventionally have a large surfacearea and a relative minimal thickness.

In general, molded parts of lactams are produced by polymerization oflactams in casting molds and, in comparison to conventional plasticelements, for example, also comprised of polyamide, are in generalharder, stiffer, and more wear resistant. This is primarily a result oftheir higher crystallinity. By predetermining the method parameters suchas temperature, residence time, etc., as well as by selecting theadditives to be used and the post-treatment, the respectivetechnological properties of the molded parts can be affected. Most ofthese aforementioned conditions, however, can only be maintained withvery great economical and technical expenditure. The high crystallinityhas primarily the effect that the shaped parts produced from lactamsalso have a high brittleness. Thus, the molded parts produced fromlactams in conventional methods can be destroyed in an explosion-likefracture by suddenly occurring overloads due to a so-called cold flow.This can result in considerable damage in connection with constructiveparts relevant to safety aspects.

Even though in the prior art the use of fiber glass-reinforcedthermosetting resins cured from monomers in molds, these prior artmethods cannot be simply transferred by a person skilled in the art.Thermosetting plastics, i.e., fiber glass plastics in the conventionalsense, are produced by radical-induced polymerization. The manufactureof molded parts with polymerization of lactams relates to thermoplasticmaterials produced by anionic polymerization. This process concernsparameters that are of no consequence in radical polymerization. Thus,the transfer of methodological knowledge of the thermosetting plasticmanufacture cannot be simply applied to the thermoplastic manufacture bylactam polymerization since the anionic polymerization is subject to itsown rules. The rules for thermosetting plastic manufacture cannot betransferred.

Special problems for the anionic polymerization of lactams result, forexample, from method parameters in regard to moisture and thesensitivity of the lactams to polar substances. For example, it is notpossible to embed conventional fiber glass plastics into the lactams,but they must fulfill certain criteria in regard to the methodparameters.

Methods for manufacturing molded parts by polymerization of lactams are,for example, known from DE 1 174 982 A1 and DE 1 910 175 A1. The firstmentioned method discloses the manufacture of cast polyamide shapedbodies which are produced by rotational casting whereby the rotationalforces may not surpass the gravitational forces. Even though the generalpossibility of introducing filler materials is disclosed, it is notrecognized here with which measures defined technological properties canbe produced. DE 1 910 175 A1 discloses also a method for producingpolyamide cast parts whereby mats are introduced for increasing thestiffness of thin-walled molded parts. There is no disclosure in regardto the directed use of fibers or the use of a certain type of fibers.

In DE 2 050 572 A1, DE 1 214 865 A1, and DE 1 066 012 A1, rotational andspinning cast methods are disclosed which describe method details but norelation to the manufacture of molded parts fiber-reinforced in adirected manner.

It is an object of the present invention to improve a method formanufacturing molded parts by polymerization of lactams in molds suchthat the resistance of the inventively produced molded parts withrespect to suddenly occurring load peaks is increased and thereproducibility of the defined technological properties of theinventively produced molded parts is improved.

SUMMARY OF THE INVENTION

The technical solution to this object is a method for manufacturingmolded parts by polymerization of lactams in molds in which the fibermaterial is introduced into the mold part for embedding in the moldedpart before the polymerization step takes place and is stationarilypositioned relative to the mold.

The inventive method for manufacturing a sheet molded part bypolymerization of lactams in a mold is characterized by the steps of:

placing a fiber composite material into a sheet mold having large planesurfaces;

fixedly positioning the fiber composite material in the sheet mold;

introducing the lactams into the sheet mold;

polymerizing the lactams.

The method further comprises, before the step of introducing, the stepof heating the sheet mold to a temperature of 120° C. to 190° C.

The method further comprises, before the step of introducing, the stepof heating the sheet mold to a temperature of 145° C. to 180° C.

The method may comprise, before the step of placing, the step ofproducing the fiber composite material from fiber glass. The surface ofthe fiber glass of the fiber composite material is preferably free ofionic substances.

The method may further comprise the step of selecting a proportion offiber glass in the sheet molded part to be 10-70%.

The method may further comprise, before the step of placing, the step ofproducing the fiber composite material from plastic fibers.

The method also comprises, before the step of placing, the step ofproducing the fiber composite material as a needled felt or needlednonwoven.

The method may include, before the step of placing, the step ofproducing the fiber composite material as rovings.

The method may include, before the step of placing, the step ofproducing the fiber composite material as a woven material (fabric).

The method may also comprise, before the step of placing, the step ofproducing the fiber composite material as mats.

The method may include, before the step of introducing, the steps ofadding additives to the lactams and melting the lactams and additivesfor forming a melt.

In the step of melting, the lactams and the additives are heated to atemperature of 110° C. to 140° C., preferably to a temperature of 116°C. to 125° C.

The step of introducing includes pouring the melt into the sheet moldand the method may further comprise, after the step of pouring, the stepof closing the sheet mold and the step of evacuating the sheet mold.

The step of introducing includes pouring the melt in batches into thesheet mold.

The method may further include, after the step of fixedly positioning,the step of heating the sheet mold.

The method may also comprise, after the step of fixedly positioning, thesteps of closing the sheet mold and flowing nitrogen through the closedsheet mold.

The method may comprise, before the step of introducing, the step ofclosing the sheet mold, wherein the step of introducing includes meltingthe lactams and injecting the lactams into the closed sheet mold.

The method may comprise the step of pressurizing the closed sheet mold.

The method may further comprise, after the step of introducing, the stepof rotating the sheet mold.

In the step of rotating the sheet mold is rotated at a speed of 30 to250 rpm.

In the step of rotating the sheet mold may be rotated at a speed of upto 2000 rpm.

The method may include, before the step of introducing, the steps ofadding additives to the lactams and melting the lactams and additivesfor forming a melt, wherein the step of introducing includes pouring themelt into the sheet mold at a center of rotation of the sheet mold.

The invention also relates to a sheet molded part, comprised ofpolymerized lactams and a fiber composite material embedded in thepolymerized lactams, produced in accordance with the inventive method.

Preferably, fiber composite material is fixedly arranged atpredetermined locations before being embedded.

According to the inventive method it is thus made possible with thedirected introduction of fiber composite material to improve thereproducibility of the technological properties of the molded partmaterial. These include, for example, tensile strength, elastic modulus,impact resistance etc. The term fiber composite material in the contextof the invention includes rovings, fabrics, mats, nonwovens and felt,i.e., according to advantageous embodiments of the invention, needlednonwovens and needled felt, fabrics, rovings, roving fabric, and mats,preferably of fiber glass, respectively, plastic fibers.

An important aspect is the consideration of compatibility of thematerial (matrix) with the fabric, from which the technologicalproperties result. For example, it is important that the fibers are freeof inhibiting compounds.

The fiber composite material, according to a suggestion of theinvention, is stationarily fixed in the mold so that the geometriclocation of the fiber composite material within the workpiece isensured. In an advantageous manner, the mold with the fabric introducedis heated to a temperature of 120° C. to 190° C., preferably 145° C. to180° C. According to a suggestion of the invention, the lactams arepoured into the mold after placing the fiber composite material into themold. It is advantageous in this context to melt the lactams with therequired additives before casting into the mold. Advantageously, thetemperature is adjusted to 110° C. to 140° C., preferably 116° C. to125° C.

According to an advantageous embodiment of the invention, the mold isclosed after introduction of the lactams and evacuated. In addition, oralternatively, according to another suggestion of the invention, themold during and/or after introduction of the lactams is rotated.Advantageously, rotational speeds of 30 to 250 rpm are adjusted. Atthese speeds the mold can also be rotated biaxially which isadvantageous for the manufacture of hollow bodies. This method allowsfor the batch-wise pouring of lactams so that in sequence individualcasts as a function of the viscosity of the melt can be performed. Thismethod also allows for the subsequent introduction of fiber compositematerial insofar as the required method parameters with respect to thelactam melt are observed so that complete polymerization cannot occur.

According to a further advantageous suggestion of the invention, therotational speeds are adjusted to between 100 and 2000 rpm. The castinginto the rotating mold is carried out at the center of rotation. Allnamed methods make sure that air inclusions in the manufactured moldedparts are prevented and that a good connection between the castingmaterial and the fiber composite material is achieved.

According to an advantageous embodiment of the invention glass is to beused in an amount as large as possible. This results in an increaseespecially of the stability values. It is especially important in thiscontext that the polymerized polyamide adheres as tightly as possible tothe surface of the glass, that air inclusions in the finished part areat a minimum, and that especially the polymerization is substantiallycomplete with a remaining lactam contents of maximum 2%.

According to a suggestion of the invention fiber glass fabrics areprimarily used. They have the advantage of minimum volume so that alesser amount of air is introduced into the molds. Furthermore, bylayering a higher glass proportion can be achieved. The fabrics can alsobe placed with respect to the required loading direction in differentfiber orientation in order to provide a composite that can satisfydifferent static and dynamic loads. The use of fiber glass fabrics alsoallows for the manufacture of thin-walled elements.

According to a suggestion of the invention the glass fiber fabrics areintroduced into the heated mold and the mold is then heated further.With this measure the moisture contained within the fabric can beremoved. For this purpose, it is also possible to flow nitrogen throughthe introduced fabric in the closed mold. It is also possible toevacuate the closed mold. Primarily, with these measures moisture isavoided which would impede or even inhibit the polymerization of thelactams so that a remaining lactam proportion of more than 2% wouldresult.

According to a further suggestion of the invention, the fiber glassmaterial is used which has no ionic substances at its surface. It isfurthermore suggested to perform the evacuation of the mold such thatair inclusions within the finished part are prevented. In this context,it is possible, on the one hand, to generate a vacuum after introductionof the lactams and the closing of the mold. It is also possible toinject the lactams into the evacuated mold.

According to another suggestion of the invention, the mold ispressurized during polymerization. For this purpose, the mold can beintroduced into a press. With this measures, it is possible to produceespecially tension-reduced shaped parts. According to a furthersuggestion of the invention, additives may be added to the lactams inorder to affect the desired technological properties. Conventionally,certain reactive prepolymers are used with which in the highlycrystalline cast polyamide the amorphous contents can be increased in adirected manner so that the brittleness is reduced and the tenacitystrength is increased. According to a further suggestion of theinvention, the additives are selected under the aspect of making thematerial recyclable as a thermoplastic material. Of course, no additivesshould be used which would impede anionic polymerization.

With the inventive method it is possible to produce sheet molded partswhich, while allowing an economical manufacture, have substantially thetechnological properties of sheet metal molded parts. Furthermore, themolded parts produced according to the inventive method also havetechnological properties of molded parts of thermosetting plastics, butare recyclable more simply and especially more completely.

With the inventive method it is also possible to produce molded partswith reproducible technological properties whereby, in general, theresistance in regard to suddenly occurring load peaks is increased. Themechanical properties can, for example, be affected by the type of fibercomposite material, the way of positioning with respect to the number oflayers, the fiber orientation etc.

A molded part produced according to the inventive method is novel withrespect to its construction and its technological properties. Especiallysurprising is the selectability and reproducibility of the technologicalproperties.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages and features of the invention will result from thefollowing description in conjunction with exemplary embodiments of theinventive method and with the aid of Figures. It is shown in:

FIG. 1 a schematic flow chart for explaining the method steps;

FIG. 2 a schematic cross-sectional view of an embodiment of a moldedpart, and

FIG. 3 a schematic representation of an arrangement for performing themethod.

DESCRIPTION OF PREFERRED EMBODIMENTS

The embodiments will be explained with the aid of the schematic flowchart represented in FIG. 1.

First, in the steps 1 and 2 the lactams and the required additives areprovided in predetermined amounts and are specified. The termspecification in this context means the quantitative and qualitativecorrelation of the individual raw materials to one another whichinfluence a successful performance of the method as a function of thetechnological know-how. For example, a precise specification may keepthe proportions of extractable components of the material, i.e.,monomers and oligomers, below 2% so that the technological propertieswill not be impaired long-term.

In method step 6 the fiber composite material is provided incorresponding amounts, cuts, and specifications. The fiber compositematerial can be selected depending on the desired technologicalproperties.

For example, fiber glass roving fabrics, textile glass mats, needledglass felt, or needled glass nonwovens can be introduced. When usingfiber glass roving fabrics, a homogenous connection between the matrixand fibers must be provided. The connecting points of the glass fibertextile mats can be connected with suitable thermosetting adhesives. Thefoam-like structures of felts and nonwovens support the significantincrease of impact adsorption capability of the material produced.

In method step 3, the lactams and the additives are mixed and melted. Atemperature control 4 and a time control 5 provide for generating thedesired melt with defined properties and a defined, method-specificbehavior.

The fiber composite materials provided in the method step 6 areintroduced into the mold 7. A temperature control 8 provides formaintaining the desired temperatures. As a function of the time controls9 and 10, the introduction of the lactam melt and, as a function of theprocess control 11, the subsequent polymerization take place.

The process control includes, for example, the closure and evacuation ofthe mold after completed introduction, the rotation of the mold forproducing a spin cast, whereby rpms between 100 and 2000 rpm areprovided. The casting occurs at the rotational center. For a rotationalcast, for example, a biaxial cast, the rotational speed is adjusted to30 to 250 rpm. This method allows a batch-wise introduction as afunction of the increase of viscosity of the melt in the mold.

After completed polymerization the finished work piece can be easilyremoved from the mold.

After removal, the desired post-crystallization effects can be producedby storing the workpieces in a defined atmosphere, for example, in aheated or air tight atmosphere.

The mold can be preferably made of stainless steel. Parting agents arenot required.

As is shown in FIG. 2 by example of a cable pulley, by stationarilyfixing the fiber composite material within the mold a defined placementof the fiber composite material in the molded part can be effected sothat the defined technological properties will result. The finishedmolded part 12 is comprised of a lactam body 13 in which the fibercomposite material, in the shown embodiment fiber mats 14, are embeddedin a defined manner.

The mold 15 represented schematically in FIG. 3 has a cavity 16 forproducing a sheet molded part. In the shown embodiment, a fibercomposite material 17, for example, a fiber glass fabric is placed intothe cavity 16. To the cavity 16 a mixing head 18 is connected so that inthe shown embodiment a lactam melt can be introduced by the so-calledinjection method. Also connected to the cavity 16 is a vacuum pump 19which produces a vacuum within the cavity 16 so that the melt is suckedvia the mixing head 18 into the cavity 16. The fiber composite materialplaced into the cavity 16 is thus embedded into the melt beforepolymerization.

The present invention is, of course, in no way restricted to thespecific disclosure of the specification and drawings, but alsoencompasses any modifications within the scope of the appended claims.

What we claim is:
 1. A method for manufacturing a sheet molded part bypolymerization of lactams in a sheet mold, said method comprising thesteps of:placing a fiber composite material into a sheet mold;stationarily fixing the fiber composite material in the sheet mold in adefined geometric location; introducing the lactams into the sheet mold;polymerizing the lactams to form the sheet molded part having embeddedtherein the fiber composite material in said defined geometric location.2. A method according to claim 1, further comprising, before the step ofintroducing, the step of heating the sheet mold to a temperature of 120°C. to 190° C.
 3. A method according to claim 1, further comprising,before the step of introducing, the step of heating the sheet mold to atemperature of 145° C. to 180° C.
 4. A method according to claim 1,further comprising, before the step of placing, the step of producingthe fiber composite material from fiber glass.
 5. A method according toclaim 4, wherein a surface of the fiber glass of the fiber compositematerial is free of ionic substances.
 6. A method according to claim 1,further comprising the step of selecting a proportion of fiber glass inthe sheet molded part to be 10-70%.
 7. A method according to claim 1,further comprising, before the step of placing, the step of producingthe fiber composite material from plastic fibers.
 8. A method accordingto claim 1, further comprising, before the step of placing, the step ofproducing the fiber composite material as a needled felt or needlednonwoven.
 9. A method according to claim 1, further comprising, beforethe step of placing, the step of producing the fiber composite materialas rovings.
 10. A method according to claim 1, further comprising,before the step of placing, the step of producing the fiber compositematerial as a woven material.
 11. A method according to claim 1, furthercomprising, before the step of placing, the step of producing the fibercomposite material as mats.
 12. A method according to claim 1, furthercomprising, before the step of introducing, the steps of addingadditives to the lactams and melting the lactams and additives forforming a melt.
 13. A method according to claim 12, wherein in the stepof melting the lactams an d the additives are heated to a temperature of110° C. to 140° C.
 14. A method according to claim 12, wherein in thestep of melting the lactams an d the additives are heated to atemperature of 116° C. to 125° C.
 15. A method according to claim 12,wherein the step of introducing includes pouring the melt into the sheetmold, the method further comprising, after the step of pouring, the stepof closing the sheet mold and the step of evacuating the sheet mold. 16.A method according to claim 12, wherein the step of introducing includespouring the melt in batches into the sheet mold.
 17. A method accordingto claim 1, further comprising, after the step of fixedly positioning,the step of heating the sheet mold.
 18. A method according to claim 1,further comprising, after the step of fixedly positioning, the steps ofclosing the sheet mold and flowing nitrogen through the closed sheetmold.
 19. A method according to claim 1, further comprising, before thestep of introducing, the step of closing the sheet mold, and wherein thestep of introducing includes melting the lactams and injecting thelactams into the closed sheet mold.
 20. A method according to claim 1,further comprising the step of pressurizing the closed sheet mold.
 21. Amethod according to claim 1, further comprising, after the step ofintroducing, the step of rotating the sheet mold.
 22. A method accordingto claim 21, wherein in the step of rotating the sheet mold is rotatedat a speed of 30 to 250 rpm.
 23. A method according to claim 21, whereinin the step of rotating the sheet mold is rotated at a speed of up to2000 rpm.
 24. A method according to claim 21, further comprising, beforethe step of introducing, the steps of adding additives to the lactamsand melting the lactams and additives for forming a melt, wherein thestep of introducing includes pouring the melt into the sheet mold at acenter of rotation of the sheet mold.
 25. A sheet molded part, comprisedof polymerized lactams and a fiber composite material embedded in thepolymerized lactams, produced in accordance with the method of claim 1.26. A sheet molded part according to claim 25, wherein said fibercomposite material is fixedly arranged at predetermined locations beforebeing embedded.